It's taken four years of hard work, but Robert Downey Jr. is now at the top of his game, he tells Stephanie Bunbury.

Robert Downey Jr. describes himself, at least in a roundabout way, as obsessive. Now there is a line that rings true. Picture the world's most cleaned-up dad waiting for his son to come over for his regular Wednesday just a few days before he is due to appear in his primary-school production of As You Like It. He's a man whose gimlet focus, at the age of 41, is perhaps a little out of proportion to the demands of everyday life, who snaps with tension rather too readily. It is also, however, a cheering picture of someone who knows himself, as we all should, to be vaguely ridiculous.

"I said, 'Right, Wednesday night you are going to stay over and we're going to run lines'," says Downey, his voice rising instantly to a remembered level of urgency. "He goes, 'Cool'. He comes over Wednesday and I am like prepped, ready to do my Actors' Studio thing and he says, 'You know what? I kind of know my lines, but now, my iPod . . ."'

He rolls his eyes: can we believe this? "And I'm like, 'F--- your iPod!"'

Downey is on a roll now, elated with the story, while everyone stares at him: this is how he does fathering?

"He goes, 'I want to download some stuff - it will only take 10 minutes!' And I'm saying, 'Nothing takes 10 minutes! If anything only took 10 minutes, everyone would do, like, a million things a day!"'

Stage dads, eh? On the whole, however, this perfectionist approach to the demands of the iambic pentameter is a whole lot healthier than the obsession for which Downey became infamous. He has been clean for more than four years now. He is also back with a vengeance, stacking up great performances as if he is trying to make up for lost time. Which, of course, he is.

Back in 1992, Downey won an Oscar for his effervescent performance as Chaplin in Richard Attenborough's film of the same name. He had also just married Deborah Falconer and was universally regarded as a dazzling talent on an infinite rise. His son Indio was born in 1993. Then the cycle of arrests and broken probations began. His marriage fell apart. From the long distance of celebrity watching, Downey's life seemed wild, maverick, a carnival of excess: it made for great stories. There were some fine performances in there as well, too - in Short Cuts, The Gingerbread Man and Wonder Boys.

"If you have good work in you, it's going to come out no matter how much you try to stuff it down, which is why guys like Poe and Hemingway and Philip K. Dick and Hunter S. Thompson were prolific and why we still find them worthwhile," says Downey now.

He still doesn't know why he spent all that time "rolling around in sewage"; he doesn't blame anything but his own contrariness for the wasted time. "There is no good reason why I couldn't have been happy, healthy, productive, wealthy and successful all along. Those were my shortcomings."

But there you have it: these things happen.

"It's a really tough and shitty road to be obsessive about anything," he says. "It's a huge deal. Then again, if you talk to anyone at the leading edge of modern psychiatry or just philosophy, anyone who really understands human behaviour, they say well, it's perfectly normal for people to have a fixated, obsessive relationship with something for a period of time.

"And then, you know what? It just stops. Things go crazy; they get chaotic and the end result is sometimes tragic, but more often than not people just say, 'Oh, that was an incredibly long and mysterious f---ed-up phase'."

After a few fallow years, he got lucky. In 2002, Elton John checked him out of rehab for a day to star in his video I Want Love. The breakthrough came with Kiss Kiss, Bang Bang. Then George Clooney cast him as a reporter in his Cold War drama about free speech, Good Night, and Good Luck.

"I'm a cockroach. I'm really resilient," Downey agreed at the time. "If I had to resort to eating soda cans, I'd still feel pretty good about it."

Last year, he married Susan Levin, who produced Kiss Kiss, Bang Bang. Marriage - to the right person, as he says - has helped.

"Accountability is good. But I think the most important ingredient is humility, which is just an honest assessment of what is really happening. Hopefully, we mature to the point where we don't regret the past. Strange as it may seem, I don't, but that's because it brought me to where I am."

Here's where we are: in Cannes, discussing his role in A Scanner Darkly, Richard Linklater's curious animated adaptation of Dick's futuristic novel in which real actors' performances have been turned into graphics. Thus, Downey, along with Keanu Reeves, Winona Ryder and Woody Harrelson, look exactly as they do in life, but as a line drawing. The idea, as director Linklater explains it, is to evoke the characters' own confusion by hitting a mark somewhere between reality and something else.

"I felt the way it works on the viewer's brain is similar to what they are going through, particular the dissonance it creates in the viewer. Is this real? If not, what is it?" he says. "It makes you question your reality, which is a big thing of Philip K. Dick's."

Which is also what the story is about. Reeves plays an undercover police agent assigned to penetrate the pervasive culture revolving around Substance D, a new and powerful drug that stimulates psychotic levels of paranoia in users. In order to reach the heart of this world, he must become addicted himself, while living in the D equivalent of a crack-house. Downey is a wisecracking crazy who, it gradually becomes clear, is intent on betraying his supposed friends to save his own junkie skin.

People did double-takes, says Linklater, when he listed his cast. All that trouble, waiting to happen. "They would say, 'Whoa, what was that like?', because they'd heard stories. But it was a very healthy set, you know. Robert and Woody were doing their Pilates and eating raw food. And we finished a day and a half early, which shows how much trouble they gave me."

Just after finishing A Scanner Darkly, Downey began work with director David Fincher on Zodiac, a true-life murder thriller, playing an investigative reporter. Yet again, the part could melt into the real-life persona. Reporter Paul Avery, says Downey, "is renowned for his heavy boozing, coke-snorting, acid-dropping proclivities."

More typecasting, perhaps, but from the inside, says Downey, it didn't feel as if this had anything to do with him.

"It wasn't like stirring shit up. I'm working with Fincher, so we're doing 75 takes of picking up a f---ing telephone. I'm not thinking, 'God, this reminds me of that tough period in my life'. There's no time."

And two more Downey films are in the starting blocks. A Guide to Recognizing Your Saints is Dito Mondiel's autobiographical film about the friends who died around him in his youth - more drug stories - whom he became convinced were protected by saints. Downey plays the adult Dito.

Next year, we will also see Steven Shainberg's Fur, an imaginary story centred on the real-life photographer Diane Arbus, portrayed by Nicole Kidman. Three further films are in the works, including Paramount's new superhero title Iron Man, with the once-uninsurable Downey as the lead.

When you remember the Downey who couldn't seem to stay out of jail, you wonder how he got here.

"You go to bed at night," says the man himself, sardonically. "You get up in the morning. It's a f---ing miracle."

Unlike the rest of Cannes, he says smugly, he isn't nursing a red-wine hangover.

"I put down all those survival tools. I'm just roughing it now."

So does he ever miss it, that edgy business of self-abuse?

"Hell no!" says Clean Dad, astonished. "Miss what? I'm so on top of my game! Not just my career. Career is one part of an overall life - and it's life that's of interest to me now."